Glycoside – Definition, Mechanism and Uses

What is a Glycoside?

A glycoside is a chemical compound consisting of two main components: a sugar portion (glycone) and a non-sugar portion known as the aglycone or genin. The two parts are connected via a glycosidic bond. Glycosides occur in great diversity in nature, especially in plants, but also in animals and microorganisms.

Depending on the type of aglycone and the attached sugar, glycosides differ considerably in their chemical structure, properties, and biological activity. Many glycosides are pharmacologically active and are used as active pharmaceutical ingredients in medicine.

Types of Glycosides

Cardiac Glycosides

Cardiac glycosides (cardenolides) are among the most medically significant glycosides. They are derived from plants such as foxglove (Digitalis purpurea and Digitalis lanata). The most well-known representatives are digoxin and digitoxin. They act directly on the heart muscle and are used to treat heart failure and certain cardiac arrhythmias.

Flavonoid Glycosides

Flavonoid glycosides are found in many fruits and vegetables. They possess antioxidant, anti-inflammatory, and vascular-protective properties. A well-known example is rutin, found in buckwheat and citrus fruits.

Saponins

Saponins are a group of glycosides with soap-like properties. They are found in legumes, quillaja bark, and licorice root, among others. Saponins can influence the absorption of certain nutrients in the gut and show potential immunomodulatory and cholesterol-lowering effects.

Cyanogenic Glycosides

Cyanogenic glycosides such as amygdalin (found in bitter almonds and apricot kernels) can release hydrogen cyanide in the body and are toxic in higher quantities. They occur in numerous plants as a natural defense against herbivores.

Glucosinolates

Glucosinolates are sulfur-containing glycosides found mainly in cruciferous vegetables such as broccoli, cabbage, and mustard. Their breakdown products (e.g., isothiocyanates such as sulforaphane) are being intensively studied for their cancer-preventive properties.

Steroidal Glycosides

Steroidal glycosides contain a steroid aglycone and are also found in various plants. Some are used as starting materials for the synthesis of steroid hormones and corticosteroids.

Mechanism of Action

The mechanism of action of glycosides depends strongly on the respective aglycone. In the case of cardiac glycosides, they inhibit the sodium-potassium ATPase (Na⁺/K⁺ pump) in cardiac muscle cells. This leads to an increase in intracellular sodium concentration, which in turn causes a rise in intracellular calcium levels via the sodium-calcium exchanger. The result is an increased contractile force of the heart (positive inotropic effect) and a slowing of the heart rate (negative chronotropic effect).

The sugar portion of a glycoside significantly influences the solubility, bioavailability, and absorption of the compound in the organism. After enzymatic or acid-catalyzed cleavage of the glycosidic bond, the aglycone is released and exerts its biological effect.

Medical Applications

Glycosides are used in various areas of medicine:

• Heart failure and atrial fibrillation: Cardiac glycosides such as digoxin are used to strengthen heart muscle contractions and regulate heart rhythm.
• Antioxidant protection: Flavonoid glycosides act as antioxidants and protect cells from oxidative stress.
• Anti-inflammatory effects: Various plant-derived glycosides demonstrate anti-inflammatory properties.
• Cholesterol reduction: Saponins can reduce cholesterol absorption in the intestine.
• Cancer prevention (research area): Glucosinolate breakdown products are being investigated for their potential role in cancer prevention.

Safety and Side Effects

The safety of glycosides varies considerably depending on the type and dosage. Cardiac glycosides have a narrow therapeutic window, meaning the difference between an effective and a toxic dose is small. Possible side effects of overdose include:

• Nausea, vomiting, and diarrhea
• Cardiac arrhythmias (potentially life-threatening)
• Visual disturbances (e.g., yellow vision)
• Confusion and dizziness

Cyanogenic glycosides can cause cyanide poisoning when raw foods such as bitter almonds or apricot kernels are consumed in excess, making them hazardous in larger quantities.

Occurrence in Foods

Many glycosides are a natural part of our diet. They are found in the following foods:

• Fruits and vegetables: Apples, cherries, broccoli, onions (flavonoid glycosides)
• Legumes and grains: Soy, chickpeas, oats (saponins)
• Cabbage and cruciferous vegetables: Broccoli, Brussels sprouts, mustard (glucosinolates)
• Bitter almonds and fruit kernels: (Cyanogenic glycosides – consume with caution)

References

1. Mutschler, E. et al. – Mutschler Drug Effects: Pharmacology, Clinical Pharmacology, Toxicology. 10th edition. Wissenschaftliche Verlagsgesellschaft, Stuttgart, 2012.
2. Bruneton, J. – Pharmacognosy, Phytochemistry, Medicinal Plants. 2nd edition. Lavoisier, Paris, 1999.
3. World Health Organization (WHO) – Traditional Medicine Strategy 2014–2023. WHO Press, Geneva, 2013. Available at: https://www.who.int